Method for producing a cooling element, and a cooling element

ABSTRACT

A method for manufacturing cooling elements for power electronic components and other cooling devices for electronic appliances, wherein molten metal is fed into a closed mould including a core part and a mould part. The metal is allowed to become solid whereafter the finished casting is removed from the mould. An insert package is assembled of lamellas made from a material which at least essentially corresponds to the casting material by pressing together several lamellas arranged next to one another such that an air slot remains between the lamellas arrranged next to one another such that an air slot remains between the lamellas. This insert package is machined such that it fits into a recess in the core part and the insert and the insert package is placed in the recess in the core part before closing the mould, whereby the package is fastened to the casting once the mould has been filled.

This application is the national phase under 35 U.S.C. §371 of PCTInternational Application No. PCT/FI98/00611 which has an Internationalfiling date of Aug. 3, 1998, which designated the United States ofAmerica.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to method according to the preamble ofclaim 1 for manufacturing cooling elements for power electroniccomponents and other electric appliances.

The invention also relates to a cooling element manufactured accordingto the invention.

Description of Background Art

Cooling elements for electric appliances are most commonly made of long,extruded aluminium profile cut into suitable lengths. By extruding, itis easy to produce many kinds of profiles, wherefore it is simple toproduce a suitable profile for different intended uses, and in thismanner, a large number of different kinds of cooling elements can bemanufactured. However, elements made from extruded profile have a numberof disadvantages relating particularly to the cooling of powerelectronic components. With modern components, high electric power canbe handled with relatively small equipment. When such high power is ledthrough a semiconductor component, naturally, power loss results,heating the component. This heat is generated within a small area andair cooling is usually employed to remove it. The heat transmissioncapacity of an air-cooled cooling component depends especially on thearea of the surface to be cooled and on the distance from the heatsource to the surface, i.e. the length of conduction. As the size of theelement is as good as always restricted due to structural reasons, thecooling capacity of a cooler of certain dimensions depends on how largean area to be cooled can be packed into a certain volume. The coolingcapacity can be essentially increased by means of a forced air flow, butthe element must hereby be designed such that it does not prevent theflow of cooling air to the other elements.

Cooling elements are also manufactured by casting. Die casting, forexample, can be applied to effectively and at a reasonable costmanufacture large numbers of cooling elements, wherefore this is anadvantageous method for the series production of similar elements neededin large numbers. The casting can be shaped more freely than an extrudedprofile but because the fins of the cooling element must be shaped bymeans of the mould and cores placed in the mould, the maximum density ofthe fins as well as the thickness of the fins are restricted. All partsof the casting must have a certain cross-section to enable the fillingof the mould and to prevent deformations occurring while the castingbecomes solid from destroying the casting. The casting tool mustwithstand several castings and a hot casting alloy, wherefore there mustnot be any very thin surfaces in the mould, such surfaces being subjectto rapid wear due to the stresses affecting the mould. Therefore, it isnot possible to apply the present casting methods to manufacture coolingelements having a high fin density.

Cooling elements must be made of a material having good heattransmission properties. Thus, copper would constitute a good rawmaterial for cooling elements but it is expensive and heavy, whereforecooling elements are usually made of aluminium-based light metal alloys.As a rule, light metal alloys have good castability properties and areparticularly suited for pressure die-casting.

SUMMARY AND OBJECTS OF THE INVENTION

The aim of the present invention is to provide a method formanufacturing cooling elements having a high fin density using pressuredie-casting techniques.

The invention is based on placing an insert assembled of lamella sheetsin the core of the casting mould and fixing the body of the coolingelement to the lamella package by pressure moulding, whereby an integralelement comprising a body and an array of cooling fins is formed.

In more detail, the method according to the invention is characterizedby what is stated in the characterizing part of claim 1.

The invention offers considerable benefits.

The method according to the invention makes it possible to manufacturean array of cooling fins having a considerably higher density than onemanufactured by conventional pressure moulding. This is of utmostimportance because the ratio of the power handled by the powerelectronic components to the size of the components is on constantincrease, wherefore a great cooling capacity must be fitted into acompact space. The body of the element is tightly adhered to the lamellapackage and thus, no air passage hampering the transmission of heat isformed between the body and the lamella package. Under advantageousconditions the surface of the lamella package can be melted by the aidof the casting material, thus forming a completely seamless piece. Theinsert and the casting material are of the same material or alloys of atleast essentially similar type, preventing the generation of stressesdue to the different thermal expansion of the materials in the casting.The production method is easily automated.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and of the scope of the invention willbecome apparent to those skilled in the art from thie detaileddescription.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention is described in more detail withreference to the annexed drawings.

FIG. 1 is a perspective view of a cooling element according to theinvention.

FIG. 2 is a top view of the element of FIG. 1.

FIG. 3 depicts a single cooling fin.

FIG. 4 is a cross section of the core part of the mould used in theproduction of the element, an insert package consisting of the fins ofFIG. 1 having been placed therein.

FIG. 5 is a perspective view of the core part of FIG. 4.

DETAILED DESCRIPTION OF THE INVENTION

The cooling element shown in FIGS. 1 and 2 consists of a body part 1 andan insert package 2 integrally connected thereto by casting, the packageforming the cooling fins of the element. The cooling element is abox-like piece whose body 1 is along its edges encircled by a hem 3forming the walls of the box. The insert package 2 is in the middle ofthe box and the upper surface of the package 2 is roughly at the heightof the edges of the hem 3. The hem part 3 is provided with an opening 4for the circulation of cooling air. As the invention does not concernthe outer shape of the cooling element, only one possible constructionis provided as an example here. It is clear that the invention can beapplied to cooling elements of varying shapes.

The invention is based on connecting a prefabricated insert package 2 toa body 1 of the same or essentially the same material by casting. Theinsert package 2 is produced by pressing together lamellas 5 made fromextruded aluminium profile to form a uniform block by means of a pressconnection. FIG. 3 depicts the structure of one lamella sheet. Thislamella sheet 5 has toothings 6, 7, 8 at both of its edges, designedsuch that the teeth on opposite sides of the lamella intermesh, allowingthe arrangement of lamellas next to each other such that the toothing ofan adjacent lamella 5 fits the gaps between the teeth of the nextlamella 5. The longest toothing 6 is provided at the edge which formsthe bottom 9 of the insert package 2 and in addition, short toothings 7,8 are provided in the middle of the lamella 5 and at the opposite edge.The insert package 2 is made of these lamelias 5 by pressing together adesired number of lamelias 5 arranged next to each other, whereby theyare fastened to each other by means of a press connection. After havingbeen joined together the lamella package and particularly its bottom 9are meticulously machined to fit the core part 10 of the casting mould.

A cross-section of the core part 10 is shown in FIG. 4 and a perspectiveview in FIG. 5. What is essential in casting the element is that thejoint between the core part 10 and the insert package must be sealedsuch that no molten casting metal can flow into the recess 11 for theinsert package 2 and between the cooling lamellas. In the case of thecore part 10 shown here, the sealing is carried out in three steps. Thelamella part 12 of the insert package is machined to have a slightlytapered shape whereby it is tightly pressed into the rectangular recess.The coning angle must be small so as not to impair the detachment of afinished piece and even a small angle is sufficient to allow a goodsealing effect. In this example the coning angle α is 1.0°. The bottom 9of the insert package is machined to have a step in that it has a borderwhich is broader than the lamella part 2. The core part 10 has acorresponding step-shaped recess into which the machined bottom part 9of the insert package 2 can be exactly fitted. The outermost edge of therecess of the core part is bevelled whereby the outer edge of the bottompart 9 forms a tight cone joint even here. The coning angle β must besmall because no such slit must be formed between the bottom 9 of theinsert package and the core part 10 into which the molten metal couldpenetrate. Further on the incoming side of the melt the bottom part isprovided with a bevelled edge 14 which facilitates the filling of themould and the flow of the melt, in this example the melt flows asindicated by the arrow A.

The casting of a cooling element is performed such that an insert isplaced in the mould, which is sealingly connected to the core part 2.The actual core part is not shown here but it is formed by a cavitywhich is shaped to correspond to the contour of the piece to be producedand which is arranged around the core part 10. Prior to the casting themould is heated to a temperature of 200° C. and closed whereafter themould is filled with a molten material corresponding to the material ofthe insert. The temperature of the cast melt is approximately 630° C.and the rate of casting is 25 m/s.

The bottom 9 of the insert is advantageously rather thick and massivebecause it must be capable of withstanding the thermal effect of themolten casting material without melting entirely. The bottom part of theabove-described cooling element has a thickness of 21 mm and a gap ofapproximately 3 mm remains between the bottom part and the wall of aclosed mould. Hereby the difference between the mass of the bottom 9 andthat of the casting material is so great that there is no danger of thebottom 9 being molten away. The temperature of the melt and the mould israised to such a high level that at least the surface of the bottom part9 of the insert melts whereby the insert becomes part of the casting asthe melt is solidified. However, a good joint is accomplished even whenno essential melting of the insert occurs.

It goes without saying that the shape of the cooling element, the corepart, the insert and the mould will vary according to the producedpiece. The casting metal alloy and the material of the insert may varybut will most advantageously correspond to each other such that theirmelting points and mixture ratios are close to each other which makes itpossible to connect the insert to the casting by melting the insertsurface. The preheating temperature of the mould as well as the castingtemperature of the melt vary according to the material cast but in thecase of aluminium alloys the temperature of the mould should preferablybe at least 200° C. and the temperature of the casting alloy 630° C.Higher temperatures will ensure the melting of the insert but, on theother hand, mould abrasion and other problems related to high castingtemperatures are increased. In addition to aluminium alloys, e.g.magnesium alloys can be used, but the casting alloy and the insert mustalways have the same primary constituent.

The above-described sealing in three steps is not necessarily alwaysneeded between the insert and the core part if one sealing methodachieves sufficient tightness. The structure of the sealing surface isto a great extent dependent of the structure of the element beingproduced.

According to the invention the insert can be arranged upside down in themoulding form. Based on simulations it would seem that the temperatureof the thinner insert end will hereby rise to a temperature which isapproximately 150° C. higher than that of the thicker insert end. In thesimulation the insert/casting interface did not undergo any strongmelting.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope for the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

What is claimed is:
 1. A method for manufacturing cooling elements forpower electronic components and other electric appliances, comprisingfabricating an insert package (2) of lamellas (5) by pressing togetherseveral lamellas (5) next to each other such that an air slot remainsbetween the lamellas, positioning the insert package (2) in a recess(11) in a core part of a mould before closing the mould, whereby thepackage is adhered to the casting once the mould has been filled, andfeeding a molten metal which at least essentially corresponds to thematerial from which the insert package of lamellas is made into theclosed mould comprising a core part (10) and a mould part, the moltenmetal being allowed to set, whereafter the finished casting is removedfrom the mould, characterized by machining the insert package (2) suchthat it fits into the recess (11) in the core part (10) and at least onemachined surface of the package (2) forms a tight joint between the corepart (10) and the insert.
 2. The method of claim 1, characterized byheating the mould to at least 200° C., the casting temperature of themelt being at least 630° C.
 3. The method of claim 1, characterized bydesigning the part of the insert package (2) which comes into contactwith the molten casting material such that it is more massive than thepart to be cast surrounding it in order to prevent the insert packagefrom melting away.
 4. The method of claim 1, characterized in that theinsert package and the casting metal alloy are of aluminium alloys. 5.The method of claim 1, characterized by producing the insert package (2)of lamellas (5) made from aluminium profile, which lamellas (5) havetoothings (6, 7, 8) at both of their edges, the toothing being designedsuch that the teeth on opposite sides of the lamella (5) intermeshwhereby the lamellas can be arranged next to one another and the teeth(6, 7, 8) of adjacent lamellas (5) fit into the slots between the teeth(6, 7, 8) of the next lamella (5).
 6. A cooling element comprising abody part (1) and an insert package (2) fastened thereto consisting ofcooling lamellas (5), wherein the insert package (2) consists oflamellas (5) made from a material which at least essentially correspondsto the material of the body part (1), the insert package (2) having beenassembled of lamellas by pressing together several lamellas (5) next toeach other such that an air slot remains between the lamellas, and thebody part (1) is cast around the insert package such that it encloses atleast part of the insert package, characterized in that the insertpackage (2) comprises at least one machined surface such that it fitsinto a recess (11) in a core part of a mold (10) and the machinedsurface of the package (2) forms a tight joint between the core part(10) and the insert.
 7. The cooling element of claim 6, characterized inthat the insert package (2) consists of lamellas (5) made from aluminiumprofile, which lamellas (5) have toothings (6, 7, 8) at both of theiredges, the toothing being designed such that the teeth on opposite sidesof the lamella intermesh whereby the lamellas can be arranged next toone another and the teeth (6, 7, 8) of adjacent lamellas (5) fit intothe slots between the teeth (6, 7, 8) of the next lamella.
 8. Thecooling element of claim 6, characterized in that the insert package (2)and the body part (1) are made of an aluminium alloy.